Can an action be mirrored?
To extend Leon Cheung's answer (my reputation isn't high enough to comment yet), you can achieve the same thing by selecting ALL of the keyframes from the Action Editor (press A) and pressing CtrlC, and then pressing CtrlShiftV to paste the flipped animation somewhere else. This works for the WHOLE animation, and can save more time than doing it manually (and is a whole lot less error prone).